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Edge HD 8 with Focuser and Reducer in Image Train – SOLUTION

Description

I recently acquired an EdgeHD 8 with the 0.7 Reducer. The objective was to have a reasonably fast f/7 SCT with some reach attached to my QSI 583wsg camera. Ideally there would be a focuser in the image train and I would not have to deal with the image shift issues with the built-in SCT focuser. However there was no current focuser that would accommodate the 105mm back-focus requirements of the reducer. That is, if the focuser was simply attached to the back of the reducer.

I called the focuser manufacturers to discuss a possible solution. Ultimately Moonlite agreed to make a prototype focuser where the reducer screwed into threads inside the body of the focuser. That way you could adjust the distance from the back of the reducer to the image sensor, in this case the QSI 583wsg to the required 105mm. This picture shows their prototype and how the components go together.

ASSEMBLY: In practice you assemble the reducer and focuser. You measure the distance from the reducer end to the face of the T-thread adapter and move the focuser so that the distance added to the distance from the face of the camera to its sensor adds up to 105mm. Then the reducer, focuser and camera is assembled.

FOCUS: Attach the image train to the back to the SCT. You might attach the reducer/focuser combination first and then add the CCD camera, it’s up to you. Now put the entire assembly on your mount and set up to do the rough focus. Point to a bright star. Use the SCT focuser to manually do the rough focus. Once that is done, lock down the SCT mirror with the two lock screws on the back of the EdgeHD. Now do the fine focus using the motorized focuser and whatever software you normally use. In this case I use the Moonlite focus motor and FocusMax. It only takes small movements to achieve fine focus. So you still maintain the 105mm back-focus from the reducer.

FULL DISCLOSURE: I have NO business relationship with Moonlite. They simply solved my auto-focuser problem. I believe that Moonlite is making more of these focusers.

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